Design Of The Courageous Mast

Design of the Courageous Mast Designers Forum By Arvel Gentry YACHTING Magazine February 1975 Arvel Gentry is a research aerodynamicist at the Co in Long Beach Cal and has long had a for sailboat aerodynamics a subject with which he hashad extensive practical experience on his successful Ranger 23Kittiwake His nonsailing weekends are spent with such December 1999design projects as a sailing performance recorder for the new 79ketch Kialoa III and mast shapes for ocean racers and 12metersHere he describes the work behind the mast section on the 1974Americas cup defender Eds The Americas Cup is defended and challenged by mensailing the most advanced boats afloat and no 12Meterskipper would want to go to the starting line with anythingbut the fastest boat and best equipment that designers andbuilders can produce Since very small differences inboatspeed can mean the margin between victory anddefeat every part of the boat hull rigging and sails isreviewed and studied for any possible improvement Much was written last summer and fall about hullshape with the disappointment of the and about sails the Kevlar mainsails and theHood versus North competition Although other design Figure 2 Aluminum shapes used to test new spar designs on the authors Ranger 23 foreground and on an Ericson 46features were less spectacular and less obvious they were background The restrictive 12Meter rule prohibits anynonetheless important A case in point was the new mast radical developments as the similarity of these shapessection shapes that appeared on both the challenger indicates The background shapes are left to right aSouthern Cross and the defender Courageous The other 12 Bergstrom and Ridder type section the section had masts with a conventional elliptical mast cross selected for Courageous and the 16year old standard 12section shape Little is known about the origins of the Meter Ellipse Cross mast shape except that it was apparently theresult of some windtunnel testing Her mast was a Although not as unusual in shape the Courageous mastmodified Dshape with the round part of the D facing was a subtle but significant variation from theforward conventional elliptical shape with a slightly blunted forward face and with two plastic strips of tiny triangles running up the forward side Figure 1 The older 12 Meter masts were extrusions from a die originally used on Columbia in 1958 To get the desired shape the Courageous mast was formed from flat aluminum plates and welded after forming Why all this effort for such a relatively small change in shape To answer that we must go back to late 1973 in Bill Fickers home Newport Beach Cal Ficker who was at that time slated to be skipper of Olin Stephens new Twelve Courageous had called together myself David Pedrick who was working for Olin and Bud Gardiner who had experience in constructing large masts Ficker wondered if a new mast section shape might lead to improved boat 1 Comparison between section shapes of the standard Being a research aerodynamicist by profession and12Meter mast and the new Courageous spar Two plastic something of a student of sailing theory I jumped at thetransition strips on the new mast delay separation of the airflow opportunity to pursue this unique problem I firstconducted a series of theoretical studies to gain a basic delay the flow separation behind the mast The of how the air flows around the mast and position of the strips was found to be different on differentsails The mast of a 12Meter forms the leading edge of the mast shapes It was also found that the optimum mastmainsail and is important to the driving force of the sail shape would be significantly different from the Courageousjust as the leading edge is important to the wing of an mast if the width and thickness proportions of the However the presence of the genoa in front of were different from those required under the strict 12the mainsail and the area of separated flow right behind Meter rulethe mast make the problem difficult to analyze The point To better hold the mast to the desired design shape theof flow separation on the mast depends upon how the air mast sections were formed from flat plates and welded inflows around the mast and this is strongly influenced by the Southern California area under the guidance of Budthe presence of the genoa and the trim of the mainsail Gardiner The three 30foot mast tubes were then shipped Next I conducted analytical studies of how the airflow to the East Coast where Hood Yacht Systems assembledand separation changed with different mast shapes I and completed the outfitting of the mastsearched for the shape that gave the least flow separation The new mast was not ready for the first set of trial raceson the critical lee side of the mast and therefore had more but went into the boat just before the July undisturbed flow on the mainsailIt was also trials However the allimportant transition strips on thedesirable to have the highest possible velocities over the lower twothirds of the mast were removed when it wasforward part of the mast and therefore the highest found that the threads in the genoa were damaged by theleading edge suction sharp front edges of the transition strips during tacking But all this was just theory I had some new ideas but maneuvers New transition strips were manufactured bythey somehow had to be tested The complicated Gardiner and reinstalled on the mast just between the genoa and mainsail and the Courageous fourrace win streak in the final trials behind the mast make accurate and meaningful Intrepid only this time the sharp edges were sanded downwind tunnel testing very difficult and expensive Wind so as not to damage the genoatunnel tests can often be just as misleading as tank tests How much was all this study and work worth in termsWell why not conduct actual sailing tests of the new mast of increased boat seed No one really knows So manyshapes other changes and improvements were being made at the The idea seemed worth trying so Steve Godshall crew same time that it was impossible to sort out anyon my Ranger 23 and I formed several experimental incremental effects However the fullscale mast sectionshapes of aluminum and wrapped them around the mast comparison tests conducted with Ficker and Gardinerof my boat and went sailing Two shapes could be tested at indicated that there should have been improvement ina time In these tests the airflow patterns were indicated by boatspeed Skipper Bob Bavier stated the no one knowsshort pieces of ribbon attached to the mast sections and how much this affected our boatspeed but I cant butmainsail and by the flow of streams of soap bubbles believe that it improved it and with the tight racing wegenerated in front of the mast were having with Intrepid any improvement however When one shape was found to have less flow small was mighty welcome when compared directly with the conventional Of course Courageous went on to defeat Southern shape it was then subjected to further theoretical in four straight races so maybe all of this mast work andstudy to find out why Sailing tests were also conducted the similar work on other gear computers etc was notwith a Dsection similar to that used on Southern Cross and really needed Well dont bet your deck shoes on it Its justwith a section similar to the Bergstrom and Ridder shape this kind of allout effort that has successfully defendedEach time the new shape was compared directly with the the Cup for 123 12Meter ellipse Figure 2 shows some of thesections tested With the help of Ficker and Gardiner the two bestshapes were then tested and compared directly with theelliptical shape under full scale conditions on a Ericson 46The mast shape that had the least flow separation in thesailing tests was selected for use on Courageous by OlinStephens The selected shape was not drastically differentbut it tested significantly better than the standard section The shape was designed to have its with the plastic transition strips on the forward face of the mast The into strips generated tiny swirls of air that changethe of the flow very close to the mast so as to 2